I spent a couple of hours sitting in my senator's office lobby the week before last, during the run-up to the healthcare "vote". I was surprised to see that the calls are answered by highly educated people (interns) sitting right there in the office. I had expected a call center in India. Ok, I suppose not really India but perhaps Indiana or at least the senator's state where wages and office space cost much less. So based on my experience the call is answered by a real person who will listen, take notes and pass on something about what you say (probably an aggregated report) to the elected official. afaik also the elected officials to pay attention (hence the whole "call your congressperson" thing). I chatted with one of these people answering calls over coffee. He told me that if there are too many calls to answer, or he has to go to the bathroom etc, you get voice mail but they do call back everyone who leaves a call-back number.

Edit: I should have said this was in the Hart Building in DC.